1. Field of the Invention
This invention concerns machines for mounting and removing a tire on and from a wheel rim, and in particular relates to the means for their operation.
2. Related Art
Known machines for tire mounting and removal on and from a wheel rim comprise an outer sheet metal casing, on the top of which there is located a rotary platform provided with self-centering means for locking the wheel rim.
To the side of the rotary platform there extends a vertical structure which supports, and locks in the required position, a horizontal arm, of which that end overlying the rotary platform supports the slide seat of a vertical arm. To the lower end of said vertical arm there is connected a tool which acts against the tire edge to urge it below the edge of the wheel rim or to extract it therefrom.
Said rotary platform is driven by an electric motor positioned within the casing via a connection shaft, between which a speed step-down gear is interposed.
In those regions in which electricity for industrial use is distributed via a three-phase alternating current grid, the electric motor used for driving the machine can be of three-phase induction or single-phase induction type.
However in those regions in which electricity is distributed via a single-phase grid a single-phase induction motor is used.
The electricity supply voltage differs for different countries, for example in Italy the three-phase a.c. supply voltage is 380V and the single-phase supply is at 220V, whereas in the United States the electricity supply is single-phase a.c. at 110V.
This requires the manufacturers of this type of machine to diversify production in accordance with the voltage and type of electricity grid in the regions in which the product is to be sold.
This is made more difficult by the fact that tire removal machines require their rotary platform to be operated with a torque which has a substantial value at the low r.p.m. at which the platform rotates when the tool is in use.
In this respect, a three-phase or single-phase induction motor is known to have a characteristic torque curve in which the torque varies substantially as the r.p.m. varies, within a very narrow range.